The role of natural gas and gas condensate fluids in petroleum and hydrocarbon production is one of increasing importance. Natural gas and gas condensates are lean fluids, i.e. they consist predominantly of the low order alkanes—methane, ethane and propane, with decreasing amounts of the higher order alkanes (butane, pentane, hexane etc.). Within a typical reservoir, these lean fluid mixtures exist at high temperatures (˜120° C.) and pressures (˜350 atm). At these conditions, the fluid is in a single-phase, similar to a dense gas. However the exists a set of (lower) temperatures and pressures at which the fluid mixture exists in two phases at equilibrium. In this two-phase condition, the liquid phase or condensate is rich in the more valuable higher order alkanes.
Natural gas and gas condensate mixtures are extremely complex as they contain a large number of unique hydrocarbon molecules. Hydrocarbons are conventionally referred by the total number of carbon atoms within the molecule and, if feasible, by a letter prefix denoting the isomer. For methane, ethane and propane (C1, C2, C3) there is one possible molecular configuration, for butane (C4) there are two molecular configurations (iso-butane and n-butane) and there are three configurations for pentane (C5). For C10 there are 75 possible configurations and for C30 there are an estimated 1-2 billion possible configurations of the hydrogen molecule. Table 1 gives a typical composition of a natural gas from Western Australia's North-West Shelf as it enters the pipeline.
The large number of different species in a naturally occurring hydrocarbon mixture makes thermodynamic prediction of its phase behaviour very difficult. The location of a gas condensate's phase boundary in the pressure-temperature (P-T) plane depends sensitively upon the small fractions of the higher order alkanes that it contains. For these reasons, experimental measurement is the only sufficiently accurate method of determining the phase behaviour of a gas condensate. Typically, the aims of such measurements in gas condensates are to determine the fluid's dew point curve and the liquid volume quality lines. FIG. 1 illustrates a typical multi-component phase diagram.
TABLE 1ComponentMole PercentageComponentMole PercentageNitrogen0.941Propane2.04Carbon Dioxide3.19Butanes0.886Methane87.04Pentanes0.205Ethane5.60Hexanes +0.106
The lines within the phase envelope are called quality lines. To the right of the critical point C, the quality lines represent the percentage of the total volume occupied by the liquid phase (1%, 5% and 10% respectively). To the left of the critical point C, the quality lines represent the percentage of the total volume occupied by the gas phase. The phenomenon of retrograde condensation, namely the increase in liquid fraction with decreasing pressure, is a significant feature of the phase behaviour of gas condensate fluids. A few of the numerous examples of the important role that accurate phase behaviour information plays in the production, processing and transport of fluid from the reservoir are discussed below.
As fluid is extracted from a natural gas reservoir, the pressure within the reservoir will drop isothermally along a path similar to abde in FIG. 1. If the pressure drops too far and the upper dew point (b) is reached, retrograde condensation within the reservoir will begin to occur. The heavier, more valuable, hydrocarbons that condense become irretrievably trapped inside the reservoir as they are adsorbed onto the reservoir rock (sandstone) by surface tension and capillary forces. Therefore, to optimise the recovery of the condensate, reservoir engineers employ gas recycling to maintain the reservoir pressure above the fluid's upper dew point pressure. Gas recycling involves the re-injection of lighter, less valuable alkanes, into surrounding re-injection wells pushing the one phase fluid in the reservoir towards the production well and maintaining the reservoir pressure above the upper dew point pressure.
It is essential that the engineers on off-shore platforms and in on-shore processing facilities understand the gas and condensate fluid's phase behaviour. Economic and efficient separation of gases and liquids, extraction of LPG, removal of CO2 and H2O from product streams, and the production of LNG all depend critically on the fluid's phase behaviour. In particular, LNG in liquid form (formed at −160° C., 1 atm) is the only viable way to export natural gas, as the ships that transport LNG have a fixed volume and only in its liquid form is there sufficient energy density in LNG to justify the cost of shipping.
Overland pipelines transporting natural gas need to ensure that the fluid is always in the single-phase, gas region. Pipelines are maintained at pressures of typically around 80 atm, and are at ambient temperatures. If during the day the condition of the fluid in the pipeline is at point P in FIG. 1, then it is conceivable that at night, the condition of the fluid may approach point Q, a lower dew point. If this lower dew point is reached and sufficient liquid forms in the pipeline, a blockage may develop. The pressure build-up on the up-stream side of the blockage will eventually cause the blockage to move, however this moving mass of liquid will have a momentum which could have potentially disastrous effects on the pipeline and/or the end consumer. Hence, knowledge of the fluid's phase envelope is essential in the design of safe and efficient natural gas transport systems.
Prior art apparatus used to measure the phase behaviour of multi-component hydrocarbon mixtures were designed primarily for use with oil mixtures. The measurements were aimed at detecting the difference between the volume of liquid oil extracted from the oil field at high pressures and the volume of liquid oil remaining at ambient conditions. The measurement of the height of the liquid gives a sufficiently accurate liquid volume, and there is no need for high resolution. However, at most conditions of interest for gas condensate fluids, the liquid volume always comprises a very small fraction of the total volume. Hence, to achieve an accurate measure of this volume, prior art PVT cells for measurement on gas condensate fluids were designed with an “hour-glass” geometry.
The prior art PVT cell system requires high-resolution volumetric pumps and a large overall volume. A sapphire window is located in the “hour-glass” section of the cell, which has a much smaller cross-sectional area than the majority of the cell. Two high-resolution volumetric mercury pumps accurately determine the total volume of the fluid and the relative positions of the bottom and top of the sample. By varying the relative positions, the liquid condensate cam always be positioned within the window and allows a highly accurate measurement of the liquid volume. However, there are significant disadvantages with the conventional PVT cell when used for the measurement of phase equilibria of gas condensate fluids. In particular, as well as being expensive, the equipment is massive, and the large samples required (typically 4000 cm3) take up to 24 hours to come to thermal and chemical equilibrium. Consequently, the measurement of only one or two data points per day is possible, and it therefore takes a long time to collect sufficient data points to develop a phase envelope. Despite their disadvantages, volumetric techniques are the only way currently available to actually measure the volume of liquid.
In 1996 Moldover et al1 developed a reentrant radio-frequency cavity resonator for automated phase-equilibria and dielectric measurements in fluids. Fluid mixtures were contained within the cavity itself, the fixed volume cavity being filled with a homogenous mixture to the desired density. The temperature was then varied while the pressure and resonant frequency were monitored. The appearance of a new phase in the cavity caused a significant change in the slope of the cavity's resonant frequency-temperature curve. This slope discontinuity is due mostly to the change in the dielectric constant of the portion of the mixture contained within a small annular gap acting as the principal capacitor of the microwave circuit within the cavity. Essentially, Moldover's instrument interrogated the vapour phase of the mixture.
Moldover's cavity clearly demonstrated that accurate and precise measurements of the phase envelope of gas mixtures could be made using microwave techniques, for a small fraction of the cost of the more traditional volumetric techniques. In addition, the cavity enabled highly accurate measurements of molecular properties, such as dielectric constants, polarisability, dipole moment, and its temperature derivative. However, Moldover's resonator had a fixed volume, and therefore different sample densities could only be obtained by varying the amount of gas in the cavity. In the case of heterogeneous, multi-component hydrocarbon mixture it is essential that the sample remain invariant and have constant composition. Furthermore, Moldover's system was only tested on a synthetic binary mixture of composition such that the discontinuous change in the vapour phase was dramatic as the dew point curve was crossed.